An Electrochemical Silver Ion Sensor

	During the course of this experiment you and the members of your group will build an electrochemical silver ion sensor based on a   Cu(s)Cu2+(aq)Ag+(aq)Ag(s)   cell with a known concentration of copper(II) ion.  The voltage produced by this cell will be used to follow the change in concentration of a solution containing silver ion.  (The solution contains the Ag+(aq) for the cell.)  The question for us to answer is "How well does the simple silver ion sensor described in this experiment function in determining silver ion concentrations?"  In order to check the performance of the sensor we will follow its potential as the concentration of silver ion changes while we titrate a solution of silver nitrate with a solution of sodium chloride.  We will then titrate a solution of sodium chloride with a solution of silver nitrate and again follow the behavior of the sensor.   

Introduction

  	A variety of electrochemical sensors are used for the measurement of pH, dissolved oxygen, concentrations of a variety of ions, and oxidation-reduction potentials.  Many of these sensors are electrochemical cells: They respond to changes in concentration of the species of interest by a change in voltage.  Applications vary widely.  These include monitoring the neutralization of waste water containing sulfides, cyanides, chromates, nitrites, or organic waste and controlling the addition of oxidants such as ozone or chlorine to drinking water, swimming pools, or cooling towers. They are used to determine the concentrations of metal ions in solutions, natural waters, and waste waters and to determine the endpoint of titrations.  Hospitals use them to monitor oxygen-rich atmospheres and divers, to monitor the oxygen content of mixtures of diving gases.
  	Electrochemical sensors contain a reference electrode and a sensing electrode.  The sensors produce a voltage that changes as the concentration or pressure of the sensed chemical species changes.  One example is a sensor used to monitor the amount of oxygen dissolved in water (a dissolved oxygen sensor) based on the following cell:

Pt  O2(aq); OH-(aq)  OH-(aq)  Pb(OH)2(s)  Pb

which involves the cell reaction

O2(aq)  +  2 H2O(l)  +  2 Pb(s)    2 Pb(OH)2(s)

The OH-  Pb(OH)2  Pb  electrode is the reference electrode: Its potential does not change.  The Pt  O2; OH- electrode is the sensing electrode: Its potential changes as the concentration of dissolved oxygen changes.  The concentration (activity) of oxygen can be determined using the Nernst equation and the difference between the standard state cell potential (Eo) of the cell and the potential (E) it develops when measuring a sample.  

E = Eo - RT/nF lnQ    where   Q = [O2]

Because we know the values of all of the variables in this equation except Q we (or a computer) can calculate [O2].

The Experiment

Building a Silver Ion Probe

1.	Completely wet a one-inch piece of cotton thread with KNO3 solution and place half of this thread inside a six-inch glass tube.  Use a small rubber septum to cap the end of the glass tube so that the thread is visible both inside and outside the tube.  See the figure.

2.	Hold the glass tube upright so that the septum is on the bottom.  Pipet CuSO4 solution into the glass tube until it is about 1/3 full.

3.	Place the electrode apparatus so the second septum is on top of the filled glass tube.  Push the copper wire through the septum and into the the tube so that a couple of centimeters of it are immersed in the CuSO4 solution.  The silver wire should be hanging outside the tube.  The probe is now assembled.

4.	Clamp the probe to a ring stand.  You will need to use a split rubber stopper to hold the narrow glass tube in place in the clamp.  You can raise and lower the electrode by raising and lowering the clamp. Be careful not to pull on the electrode's wires.

Titrating a Silver Nitrate Solution with Sodium Chloride 

All silver solutions must go into the waste jar.  All first rinses must also go into the waste jar.  NaCl solutions may go down the drain. 

1.	Label a 600 mL beaker "silver waste" and keep it at your lab station.  During the lab put all solids and waste solutions that contain silver into this beaker.  At the end of the lab period, pour the silver waste into the silver waste jar.  Also rinse the beaker with a small amount of water and pour the rinse into the waste jar.

2.	In a clean, dry, labeled beaker obtain about 80 mL of the standard NaCl solution.  Record the molarity.  Carefully rinse the buret with small portions of the solution and then fill the buret above the 0.00 mark.  Let a small amount run through the tip until the buret is on the 0.00 mL mark.  Rinses containing only a NaCl solution may discarded down the drain.

3.	In a clean, dry, labeled beaker obtain about 80 mL of the AgNO3 solution of unknown concentration.  Carefully rinse a 10.00 mL pipet with a small amount of this solution twice, putting the rinses into your waste beaker.  Repeat the rinse a second time.  Pipet 10.00 mL of the silver nitrate solution into a clean 250 mL beaker.  Using a graduated cylinder add exactly 90 mL of  DI water to the 250 mL beaker.  

4.	Place a magnetic stir bar in the diluted silver nitrate solution.  Set up the stirring apparatus and stir the solution.  Lower the silver ion probe into the 250 mL beaker containing the silver nitrate solution making sure that the stir bar will not hit it and that the NaCl solution will not drip on it.  Note that the silver wire and the thread must touch the solution in the beaker.  

5.	Make sure that the multimeter and computer are on, if they are not, call your TA.  Click on the potentiometric titration icon to open the program.  (The multimeter must be on before the program is opened, otherwise the program will not recognize it.)

6.	Perform a scout titration.  (You will find that silver chloride is insoluble.)  Record the mV reading with 0.00 mL of added titrant as your initial titration data point by clicking on the record button.  Now add 1.00 mL of titrant, change the current buret reading to 1.00 mL, and record the reading.  Make sure this data point is saved on your graph.  Continue adding titrant in this manner 1.00 mL at a time until 20.00 mL of titrant have been added. You may use the View button and Derivative Order button to view the first and second derivatives of the titration.

7.	Print the titration curve and save your data to your floppy disk by clicking on Save Data.  You will want to save both the data and the derivative.  After both of these are selected, click on Save.  Give a name to your file and save to your disk in drive A.  When the "Write File to Disk" window appears give your data a filename such as "scout1.txt". (The filename must have a  ".txt" extension.).  Single click on the down arrow next to the "Student Data" folder, then single-click "3 1/2 floppy [A:]".  Locate the endpoint on your graph.  Looking at the derivatives may help you to determine the endpoint more accurately. 

8.	Caution:  This step will erase all of your titration data.  When you have identified the exact location of the endpoint, you may prepare for another titration by clicking on Clear.

9.	Pour the beaker's contents into your waste beaker along with the first rinse.  Clean this beaker for reuse. Rinse off the silver wire and dry with an accuwipe.  Also rinse off the glass tube and blot the septum dry.  Make sure that the cotton thread is still in place.

10.	Now perform a second titration to more precisely identify the endpoint.  Set up for the titration as before.  Prepare the silver nitrate solution in the clean 250 mL beaker using 10.00 mL of the unknown and 90 mL of water as in step 3.  Refill the buret with sodium chloride.  Repeat the titration, however when you are one milliliter before the endpoint, add the NaCl solution from the buret one drop at a time until you are one milliliter past the endpoint.  Then continuing adding titrant 1.00 mL at a time until 20.00 mL of titrant have been added.

11.	Print the titration curve and save your data and the derivative to your floppy disk

12.	When your data has been saved and your graph has been printed, you may click on Clear.  Caution - once you hit Clear your data will be lost.  

13.	Rinse out your buret several times with DI water.  Drain thoroughly.  Rinse the pipet with DI water several times putting the first rinse into the waste beaker, the remaining rinses may go down the sink  Pour your beaker's contents into the waste container with the first rinse.  Rinse off the stir bar.  Rinse and blot the electrode apparatus dry.  Wipe the silver wire with an accuwipe.  You are ready to begin the second set of titrations.

Titration a Sodium Chloride Solution with Silver Nitrate

1.	Rinse the buret twice with a small portion of the unknown silver nitrate solution.  Put the rinses into the waste beaker.  Fill the buret to above the 0.00 mark with the unknown silver nitrate solution.  Let the solution flow through the tip into the waste beaker until the 0.00 mL mark is reached. 

2.	Rinse the 10.00 mL pipet twice with small portions of the standard sodium chloride solution.  Pipet 10.00 mL of NaCl into the clean 250 mL beaker.  Using a graduated cylinder add exactly 90 mL of DI water to the beaker.  

3.	Put the clean stir bar into the beaker and stir the solution.  Perform a scout titration followed by a more precise titration as before.  

4.	Save the data from both titrations to your disk.  Print out both titration graphs.  

5.	Pour the buret's contents and first rinse into the waste beaker.  Rinse all glassware with DI water.  Pour all your waste in the waste beaker into the class waste jar.  

6.	Pour the copper sulfate solution from your silver ion probe into the copper sulfate waste bottle.  Rinse the tube and copper electrode with DI water and put the first two rinses into the waste bottle.  Disassimble the sensor, rinse the bare wires and the tube several times with DI water and dry them with an accuwipe.  Rinse the several more times with DI water.

7.	Return the glass tube, septums, clean buret, stir bar, magnetic stirring apparatus, and pipet to your TA.  Make sure to clean your bench area.

The Laboratory Report

Your group laboratory report will be due at the start of the next laboratory period.  This is not a formal lab report.  Divide your report into the following sections:

1.	Title Page:  Use a title page with the title of the experiment, the names of each member of the group, course number, section number, and your TA's name.

2.	Statement of Purpose:  State the purpose of the experiment in 150 words or less.

3.	Data Section: Include your millivolt and milliter values for all four titrations and the corresponding labeled graphs of millivolts versus milliliters of titrant and first derivative curves.  

4.	Calculations:  Show each calculation.

5.	Results and Discussion:  Summarize your results and conclusions.  Address the points raised below in your discussion.

Calculations

  	Using the known molarity of the NaCl solution, calculate the molarity of the unknown silver nitrate solution and the diluted silver nitrate solution using data from both precise titrations.  Compare the values obtained from the different titrations and discuss why they might differ.  

  	Calculate the cell potential expected for a Cu(s)Cu2+(0.100 M)Ag+(X M)Ag(s)  Where X is the silver ion concentration in the 100 mL of diluted silver nitrate solution in your titration of AgNO3 with NaCl. 

  	Use the value of Ksp for AgCl and calculate the voltage expected at the equivalence point of the titration.

  	Use the value of Ksp for AgCl and calculate the voltage expected at the half-equivalence point of both titrations.

Discussion

1.	Discuss the shapes of your two precise titration curves.  How are the curves alike?  How are they different?  What does this indicate about the silver ion concentration during each titration?  Write the balanced chemical equations for each titration to support your answers.  Use the concentrations you calculated as part of your voltage calculations in your discussion.

2.	What purpose did the thread have in this experiment?

3.	The silver ion sensor is a galvanic cell.  Sketch an expanded view of the sensor and clearly label the anode, the cathode, the species in solution, the electrodes, and the salt bridge.  Indicate the oxidation and reduction half reactions.

4.	Sketch the Cu(s)Cu2+(aq)Ag+(aq)Ag(s) cell in a more traditional format like that shown.  Clearly label the anode, the cathode, the species in solution, and the salt bridge.

5.	How successful was the silver ion sensor you assembled?  What criteria are you using for your judgement?

6.	Use the World Wide Web to determine the cost of a commercial silver ion sensor.  (Search for "silver ion sensor" or "silver ion electrode".)  Include the URL of the site in your report.  What are the advantages of paying a higher cost for a commercial silver ion sensor?  When would a "home-made" sensor like the one we used today be satisfactory?